Recently, following the tendency of electronic devices towards compact size and higher mounting density, multilayer printed circuit boards have been strongly demanded, not only for industrial but also for consumer use. This type of multilayer printed circuit boards requires employing inner-via-hole connections between a plurality of circuit patterns and also requires high reliability.
A conventional method of manufacturing a double sided printed circuit board is described as follows. FIGS. 8(a) to (e) are cross-sectional views showing a method of manufacturing a conventional double sided printed circuit board. First, an insulating substrate 801 such as glass-epoxy is provided as shown in FIG. 8(a). Then, copper foils 802 are applied on both sides of insulating substrate 801 as shown in FIG. 8(b). Subsequently, insulating substrate 801 and copper foils 802 are bonded to each other through heating and pressurization. Thereafter, by using a conventional technique such as etching or the like, copper foils 802 are formed into a first circuit pattern 805 and a second circuit pattern 806 as illustrated in FIG. 8(c). Next, as shown in FIG. 8(d), through-holes 803 are drilled at places where first circuit pattern 805 and second circuit pattern 806 will be electrically connected. After that, conductive paste 804 is filled into through-holes 803 by a printing process as shown in FIG. 8(e), and the paste is hardened.
In the manner as described above, first circuit pattern 805 and second circuit pattern 806 are connected by conductive paste 804 filled into through-holes 803, and thus, a double sided printed circuit board 807 is obtained.
However, the above-mentioned conventional structure has the problem that adhesion between the conductive paste and the wall surface of the through-hole is poor. In addition, there is a gap in coefficient of thermal expansion between the conductive paste and the insulating substrate so that a boundary face of the conductive paste and the through-hole breaks due to the thermal shock of solder dipping. As a result, a failure in continuity occurs.